Atomic layer deposition (ALD) is conventionally carried out in a reaction chamber under vacuum conditions. One or more substrates are first loaded into the reaction chamber and then vacuum is evacuated into the reaction chamber and the reaction space inside the reaction chamber is heated to process temperature. The atomic layer deposition is then carried out by supplying at least first and second gaseous precursors into the reaction chamber alternatingly and repeatedly for providing a coating layer with desired thickness on the surface of the substrate. A full ALD cycle, in which the first and second precursor are supplied into the reaction chamber comprises: supplying a pulse of first precursor into the reaction chamber, purging the first precursor from the reaction chamber, supplying a pulse of second precursor into the reaction chamber and purging the second precursor from the reaction chamber. Purging precursors may comprise discharging the precursor material from the reaction chamber, supplying purge gas, such as nitrogen, into the reaction chamber and discharging the purge gas. When desired number of ALD cycles and thus a desired coating layer thickness is reached, the vacuum in the reaction chamber is vented and the substrates are unloaded from the reaction chamber. Then the same process is repeated for the next substrates.
ALD process can be modified by applying plasma to the deposition cycle, this is called plasma-enhanced ALD (PEALD). Typical plasmas used during plasma-assisted ALD are those generated in O2, N2, H2 or CO2 reactant gases or combinations thereof. In PEALD, the surface to be coated or treated with ALD is exposed to the species generated by a plasma of the reactant gas or mixture of gasses during the reactant step (=one ALD half cycle). Plasma may be capacitively created plasma in which two electrodes are placed within a small distance from each other, one of the electrodes is connected to an RF power supply and the other is grounded. Thus plasma is ignited between the electrodes. In plasma mode an electric discharge is subjected to one of the precursors such that active precursor radicals, ions, are formed from the precursor. The active precursor radicals react on the surface of the substrate during an ALD cycle.
Plasma may be created as so called remote plasma in which the active precursor radicals are formed with plasma electrodes far away from the substrate and outside of the reaction chamber. The plasma comprising the active precursor radicals is then conveyed and pulsed into the reaction chamber in a conventional manner of pulsing precursors successively. The disadvantage of remote plasma is that the life time of the active precursor radicals is very limited, typically seconds. When the active precursor radicals are conveyed from distance to the reaction chamber or to the substrate the active precursor radicals tend to lose their reactivity and to be become deactivated. When the active precursor radicals become deactivated they do not react on the surface of the substrate and thus the efficiency of the ALD coating process is decreased.
Alternatively plasma may be created as so called direct plasma in which the substrate is arranged between the plasma electrodes and the plasma discharge is arced through the substrate. In this case the plasma is ignited in the reaction space between the plasma discharge electrode coupled to RF power supply and the substrate.
Problem with prior art ALD apparatuses is that when the precursors meet each other they create film in any surface not only to the substrate to be coated. This means that film is created to the surfaces of the apparatus and this leads to an increase need of cleaning which further leads to interruptions in use and increases coating process downtime.